Revealing microscopic mechanism of vortex pinning by point defects in high-Tc Fe-based superconductor

Vortex pinning in a superconductor is critical for its application under magnetic field. However, the microscopic mechanism of vortex pinning by point defects is still unclear. Here we report a low temperature STM study on the pinned vortex in (LiFe)OHFeSe. We found that the Fe-vacancy defects in the FeSe layer can push the vortex CdGM states to high energy, which effectively reduced the energy of the vortex and thus induced the vortex pinning. Through the dI/dV spectra, we can quantitively calculate pining energy and pinning force, which agree with the value obtained by the bulk Jc measurement. Our study set up a direct connect between the microscopic vortex CdGM state and the pinning force for the first time.